union -- All elements of the first group (group1), followed by all elements
of second group (group2) not in first.

o

intersect -- all elements of the first group that are also in the second
group, ordered as in first group.

o

difference -- all elements of the first group that are not in the second
group, ordered as in the first group.

Note that for these operations the
order of processes in the output group is determined primarily by order
in the first group (if possible) and then, if necessary, by order in the
second group. Neither union nor intersection are commutative, but both are
associative.

Almost all MPI routines return an error value; C routines as the
value of the function and Fortran routines in the last argument. C++ functions
do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS,
then on error the C++ exception mechanism will be used to throw an MPI:Exception
object.

Before the error value is returned, the current MPI error handler
is called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler;
the predefined error handler MPI_ERRORS_RETURN may be used to cause error
values to be returned. Note that MPI does not guarantee that an MPI program
can continue past an error.